cated by small
letters; thus a on the index is the site of the so-called Caucasus
mountains, and similarly the Apennines are denoted by _c_. The numerous
craters are distinguished by numbers; for example, the feature on the
map corresponding to 20 on the index is the crater designated Ptolemy.

A. Mare Crisium.
B. Mare Foecunditatis.
C. Mare Tranquillitatis.
D. Mare Serenitatis.
E. Mare Imbrium.
F. Sinus Iridum.
G. Mare Vaporum.
H. Oceanus Procellarum.
I. Mare Humorum.
J. Mare Nubium.
K. Mare Nectaris.

_a._ Caucasus.
_b._ Alps.
_c._ Apennines.
_d._ Carpathians.
_f._ Cordilleras & D'Alembert mountains.
_g._ Rook mountains.
_h._ Doerfel mountains.
_i._ Leibnitz mountains.

1. Posidonius.
2. Linne.
3. Aristotle.
4. Great Valley of the Alps.
5. Aristillus.
6. Autolycus.
7. Archimedes.
8. Plato.
9. Eratosthenes.
10. Copernicus.
11. Kepler.
12. Aristarchus.
13. Grimaldi.
14. Gassendi.
15. Schickard.
16. Wargentin.
17. Clavius.
18. Tycho.
19. Alphonsus.
20. Ptolemy.
21. Catharina.
22. Cyrillus.
23. Theophilus.
24. Petavius.
25. Hyginus.
26. Triesnecker.

In every geographical atlas there is a map showing the two hemispheres
of the earth, the eastern and the western. In the case of the moon we
can only give a map of one hemisphere, for the simple reason that the
moon always turns the same side towards us, and accordingly we never get
a view of the other side. This is caused by the interesting circumstance
that the moon takes exactly the same time to turn once round its own
axis as it takes to go once round the earth. The rotation is, however,
performed with uniform speed, while the moon does not move in its orbit
with a perfectly uniform velocity (_see_ Chapter IV.). The consequence
is that we now get a slight glimpse round the east limb, and now a
similar glimpse round the west limb, as if the moon were shaking its
head very gently at us. But it is only an insignificant margin of the
far side of the moon which this _libration_ permits us to examine.

Lunar objects are well suited for observation when the sunlight falls
upon them in such a manner as to exhibit strongly contrasted lights and
shadows. It is impossible to observe the moon satisfactorily when it is
full, for then no conspicuous shadows are cast. The most opportune
moment for seeing any particular lunar object is when it lies just at
the ill